This invention relates to a method of analysis for gases and gas mixtures as well as instruments based on this method.
The instruments commonly used for gas analysis today operate with sensors which are in contact with the gas which is to be analyzed and which react more or less specifically to the components present in the gas. They usually supply electric signals from which one may conclude as to the partial pressure or the concentration of a type of gas which surrounds the sensor. In order to analyze several gases, either several sensors or highly complex measurement equipment such as mass spectrometers or spectroscopic instruments are required.
From EP 0,387,685 a method of gas analysis is known which is based on an acoustic modulation of the gas which is to be analyzed. In contrast to the method described in the following, pressure modulation is not used to utilize differing diffusion times; also the recording section of the analyzer is not a pressure sensor but a specific infra-red sensor.
From DE-GM 18 38 921 a method of gas analysis is known where the gas which is to be analyzed is linked via a molecular diffusion path to a measurement chamber in which the pressure variations are detected by means of a pressure sensor. The arising diffusion pressure is related to the concentration of a component in the gas.
However, several gases may give rise to the same diffusion pressure at suitable concentrations, a quantitative determination which is based on this method requires additional knowledge as to the composition of the gas; a general qualitative .analysis is not possible.
The present invention is aimed at a qualitative and quantitative detection of the partial pressures of a gas mixture in connection with unspecific pressure sensors. Gas analyzers which are based on this configuration shall thus be much less complex and more rugged compared to conventional multi-component analyzers. For typical single component measurements such as in helium leak detection the sensitivity of these sensors should be comparable or superior to standard instruments.
This task is solved by a method having the features of claim 1 as well as a device having the features of claims 6 and 15. Extensions offering further advantages to this result from the sub-claims. Here the composition or the pressure of the gas which is to be analyzed is first modulated (periodic modulation is preferred). After flowing through a molecular separating device, the gas produces a varying total pressure in a measurement chamber, the phase of which is characteristic for each type of gas and where the amplitudes depend on the concentrations.
The method of measurement is based on the known fact that gases which differ in atomic or molecular weight behave differently in molecular flows or in diffusion processes (refer to Wutz, Adam, Walcher, "Theorie und Praxis der Vakuumtechnik", 5th Edition, Burnswick, Vieweg, 1992 p. 28-31 and 102-106). Simplifying one may say that lighter gases generally diffuse faster through diffusion paths (capillaries, sintered bodies, filter discs, porous filters and polymers among others) compared to heavier gases. In purely mechanical diffusion paths, such as sintered bodies or porous filters, for example, the diffusion speeds of two gases relate to each other approximately according to the inverse square roots of their respective molecular weights.